1. Field of Invention
This invention relates to digital signal-rate converters. Specifically, the present invention relates to rate-converting systems, such as bandpass Digital-to-Analog Converters (DACs), for converting digital signals into output signals with desirable bandwidth characteristics, such as a bandpass signal.
2. Description of the Related Art
Digital-to-Analog Converters (DACs) are employed in various demanding applications including Direct Digital Synthesizers (DDSs), arbitrary waveform generators, and transmitters. Such applications demand circuit components and DACs that output signals with desired center frequencies and/or bandwidths.
DACs are particularly important in radar and communications systems, where transmitters must often transmit relatively high Radio Frequency (RF) signals derived from relatively low-speed digital signals. Such systems often employ transmit chains with DACs that convert digital baseband signals to analog RF signals in preparation for wireless transmission. Accompanying processor and memory speed constraints often necessitate relatively low-speed digital DAC input signals. The low-speed digital input signals may yield relatively low-frequency analog DAC output signals. The low frequency analog output signals must often be upconverted, via analog mixers and filters, to relatively high-frequency RF signals. The requisite mixing stages, which often include baseband-to-IF mixers, IF-to-RF mixers, and various filters, increase system costs and complexity.
Alternatively, DAC alias frequency regions may be employed to yield desired RF output signals from relatively low-speed digital input signals. Unfortunately, this approach generates distortion, necessities difficult filtering, and provides a relatively poor frequency response.
Generally, conventional DACs have limited bandpass capabilities and cannot provide analog output signals centered at desired bandpass frequencies, such as RF. Accordingly, existing DAC operations often require signal frequency translation via filtering and mixing either before or after the DAC. Requisite DAC interfacing may limit the speed of the DAC and consume excess power.
Hence, a need exists in the art for a DAC that can selectively convert a digital signal into an analog signal with a desired center frequency without the need for additional mixers and filters to implement frequency translation.